1. Field of the Invention
The present invention relates to phase-change recording-system optical disks (hereinafter referred to as phase-change optical disks) and optical disk apparatuses for reading user data in accordance with a synchronization mark recorded in a recording region by using a random shift.
2. Description of the Related Art
In phase-change optical disks, a mark is recorded by making a laser-spot-section amorphous. In order to prevent deterioration of a medium due to repetitive recording, that is, in order to increase the number of times a recording film can be overwritten, the start position for writing data is randomly shifted. This technology is referred to as “random shift”.
In known phase-change optical disks, only a few bytes are prepared for a sync byte as a data sync, which is provided to acquire byte synchronization when reading data, after a gap preceding a recording area in which data is written using a random shift. When the sync byte cannot be detected due to a defect of the optical disk, to dust, or to a scratch on the optical disk, byte synchronization in accordance with the random shift cannot be acquired. It is thus impossible to correctly read the data from the recording area.
When the amount of random shift is increased, a wider detection window for accommodating the amount of shift becomes necessary. This may cause a problem in detecting a synchronization mark when reading data. Generally, however, greater durability can be achieved by increasing the amount of shift.
Specifically, if a known optical disk apparatus cannot detect a sync byte functioning as a data sync prior to a data area in a recording area in a sector, the known optical disk apparatus cannot acquire byte synchronization and thus cannot read data from the data area.
In order to solve the foregoing problem, for example, Japanese Unexamined Patent Application Publication No. 7-262566 proposes a phase-change optical disk in which a synchronous code VFO 3, which is provided prior to a sync byte Sync preceding a data area in a recording area in a sector, is provided including a pattern capable of acquiring byte synchronization, instead of including a single repetitive pattern as in known optical disks.
The known technology described in Japanese Unexamined Patent Application Publication No. 7-262566 forms the synchronous code VFO 3 including the pattern capable of acquiring byte synchronization. As a result, locking of synchronization by a PLL circuit, which is the primary function of the synchronous code VFO 3, may be unlocked, and hence the PLL pull-in effect may not be achieved. Depending on the position, acquisition of byte synchronization is attempted when the PLL pull-in effect is not sufficiently achieved.
When the PLL pull-in effect is not achieved by the synchronous code VFO 3, user data recorded at a position differing according to each channel bit (each recording) using a random shift in a recording area may not be correctly read from the recording area.